1. Field of the Invention
The present invention relates to a multibeam scanning method and apparatus suitable to be applied to a laser printer, a digital copying machine whose recording section is equipped with a laser printer, and the like.
2. Description of the Related Art
As is well-known, in a laser printer or a digital copying machine whose recording section has a laser printer, a laser beam emitted from a single laser source is deflected by a rotating polygon mirror serving as a scanning element, and the surface of a photosensitive drum is scanned with the deflected laser beam to thereby record an image.
To gather recording speed in such a machine, the transfer speed of image information for modulating a laser beam should be increased, as should be the rotational speed of a polygon mirror. Since, however, there is naturally a limit to an increase in these speeds, there is a limit to an improvement in recording speed using a single laser beam, too.
An image recording technique of overcoming the above drawback is proposed in, for example, Jpn. Pat. Appln. KOKOKU Publication No. 1-43294. In this technique, the surface of a photosensitive drum is scanned with a plurality of laser beams at once, and these laser beams are modulated by different items of image information thereby to record an image. Since a plurality of lines of the image are formed simultaneously using a plurality of laser beams, the recording speed can be increased by the number of laser beams without changing the speed of the rotating polygon mirror or the image information transfer rate.
The above-described machine is called a multibeam scanning apparatus. As a laser beam generation source of the apparatus for generating a plurality of laser beams, there are known an array of laser diodes arranged in one chip, as disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 1-43294 and a plurality of laser diodes which are separate from each other, as disclosed in Jpn. Pat. Appln. KOKOKU Publication No. 6-94215.
The former laser beam generation source has the advantages of both generating laser beams without varying a pitch therebetween and reducing in size. If, however, the power of generation is increased to achieve high-speed recording, there occurs a crosstalk wherein the laser beam emitted from one laser diode influences the laser beam emitted from another laser diode. The pitch between laser beams is structurally difficult to narrow and thus the beams have to be emitted with a pitch corresponding to several lines of recording. It is thus necessary that a laser beam should jump over an image portion formed by a previously-emitted laser beam in which direction the photosensitive drum moves. The former therefore has a drawback of complicating an image forming process.
The latter laser beam generation source is capable of generating a laser beam at higher power and shortening the wavelength of the laser beam to record an image with high precision, without causing any crosstalk; on the other hand, it has drawbacks of making it difficult to set the pitch between laser beams to a desired value and causing the pitch to vary due to environmental temperature, heat generated from a laser diode itself, and the like, since the separate laser diodes have to be incorporated into an optical unit.
Since a laser scanning optical system is generally constituted as a magnifying optical system which magnifies an image several tens of times from each of laser beam generation sources to the surface of a photosensitive drum, the laser beam generation sources need to be positioned with precision of submicron. It is actually however impossible to secure such precision. Therefore, the multibeam scanning apparatus using the separate laser diodes includes a beam pitch correction control mechanism as described in Jpn. Pat. Appln. KOKOKU Publication No. 6-94215.
In the beam pitch correction control mechanism, before a plurality of laser beams are deflected by a rotating polygon mirror, a laser beam is split into beams by an optical means such as a beam splitter, and a pitch between the beams is detected, thus controlling the positions of the beams. In this case, however, there often occurs a shift in beam pitch on the surface of the photosensitive drum. The multibeam scanning apparatus is so constituted that a laser beam is deflected by the rotating polygon mirror and radiated to the surface of the photosensitive drum through an optical element such as an f-.theta. lens. If the optical element is formed of an inexpensive plastic lens or the like, the positions of laser beams radiated to the surface of the photosensitive drum will be varied due to irregularities of the respective elements, environmental humidity, etc. Consequently, even though a pitch between laser beams is detected to control the positions of the beams before the beams are deflected by the polygon mirror, the pitch is difficult to set to a desired value on the surface of the photosensitive drum.
The above-described drawbacks are summarized as follows.
In a prior art multibeam scanning apparatus using a laser diode array as a laser beam generation source, a pitch between laser beams can be determined with high precision; however, it is difficult to increase in power for high-speed recording and shorten the wavelength of each laser beam for high resolution, and also necessary to perform an interlaced scanning accompanying a complicated control. On the other hand, in another prior art multibeam scanning apparatus using each of a plurality of separate laser diodes as a laser beam generation source, it is possible to increase in power for high-speed recording and shorten the wavelength of a laser beam for high resolution, but a beam pitch is very difficult to be set to a desired value and it is varied due to environmental temperature, humidity, and the like.